1. Field of the Invention
The invention generally concerns lamp ballasts which are responsive to a phase angle controlled AC input to control the illumination level of gas discharge lamps. More particularly, the invention concerns an improved topography and dimming interface for a dimming ballast.
2. Description of the Prior Art
Lamp ballasts are known in which dimming of gas discharge lamps, typically fluorescent lamps, is responsive to phase angle control of the alternating current ("AC") power line input. Phase angle control involves the clipping of a portion of each half cycle of the AC sinusoidal power line voltage. A common type of phase angle controller, known generally as a forward phase dimmer, clips or blocks a portion of each positive or negative half cycle immediately after the zero crossing of the voltage. An example of a forward phase dimmer is the well known triac dimmer. Another type is the reverse phase dimmer, commonly known as an electronic dimmer, which passes the portion of the half-cycle immediately after the zero crossing and blocks the portion of the half cycle before the zero-crossing. In both types, the portion or angle of the half cycle which is blocked is adjustable.
JP-116698 discloses a ballast which responds to a phase controlled AC signal to vary the light level of a gas discharge lamp. The ballast has a pair of power line inputs which receive the AC mains voltage to power the ballast. The mains voltage is not phase controlled. A separate dimming input receives the hot dimmed output of the phase controller, which is the phase controlled AC waveform. A waveform shaping circuit converts the phase controlled AC signal into (i) a pulse width modulated ("pwm") signal having a pulse width corresponding to the conduction angle of the phase controlled dimming signal or (ii) a DC signal. Because the ballast has a dim input separate from the AC power line inputs, it is a three-wire device, i.e. it has a hot and a neutral AC input for powering the ballast and a separate hot dimmed input. DGM 9014982 shows a similar three-wire ballast with a separate input for the phase controlled dimming signal.
U.S. Pat. No. 4,797,599 (Ference et al) illustrates a power control circuit employing a master or control dimmer which provides a phase controlled input signal to one or several slave dimmers. The slave dimmers may be of a higher load capacity than the control dimmer, which permits the control dimmer to control higher loads than that for which it is rated. Ference '599 also discloses an external interface circuit employing this concept for connecting a master dimmer to an existing high frequency fluorescent dimming ballast which receives a known dim signal. The inputs of the interface circuit are the hot and neutral leads of the mains power supply as well as the dimmed hot output of the master controller which contains the phase controlled AC waveform. The outputs of the interface circuit are (i) a switched hot line which either provides source voltage or removes source voltage from the dimming ballast depending on the phase delay of the phase controlled waveform applied to the input of the interface circuit and (ii) a dim signal for input to the dimming ballast. The dim signal may be one of a pwm signal, a variable voltage, variable frequency or other signal as may be required by the ballast connected to the interface circuit. The inputs to the ballast are the switched hot lead and the dim signal carrying lead from the interface circuit and the neutral lead from the power line. The above system is a three wire system similar to that disclosed by JP-116698, with the difference being that the ballast receives a switched hot output from the interface circuit and the waveform shaping for the dim signal takes place in the interface circuit rather than the ballast itself.
Incandescent lamps are typically dimmed with a triac dimmer. The above three-wire ballasts employ a dimming input signal separate from the mains power supply. Such ballasts are inconvenient in that, when replacing incandescent lamps with a fluorescent lamp fixture, their installation requires the running of the additional, dim-signal-carrying wire between the controller, which is typically mounted in a wall, and the ballast mounted in the fluorescent lamp fixture in the ceiling. This results in considerable labor costs and is an impediment to market acceptance.
Lamp ballasts are also known which are two-wire devices in which the phase cut signal is not separate from the mains supply but is carried by the "hot" power line input. These are more attractive, from an installation standpoint, than the three-wire devices. U.S. Pat. No. 4,392,086 (Ide et al) discloses a ballast fed by a two wire phase control dimmer. The ballast includes an AC-DC rectifier, a high frequency inverter and an output transformer connected to the lamp. Dimming is achieved through voltage dimming, i.e. by reason of the input voltage to the inverter being reduced as a result of the phase cutting by the external triac dimmer. U.S. Pat. No. 5,192,896 (Qin) discloses another two wire dimming ballast which employs voltage dimming. The Qin '896 ballast includes an EMI input filter feeding a rectifier that supplies a DC voltage to a push-pull parallel resonant self-oscillating inverter. An output circuit includes an isolation transformer between the lamps and the inverter. The EMI filter has common and differential mode filter functions which reduces the level of high frequency interference being fed back into the power line and also improves the power factor. Because of the EMI filter, the Qin ballast will have a somewhat better power factor than that disclosed by Ide.
U.S. Pat. No. 4,866,350 (Counts) shows another two wire ballast responsive to a dimmer. The ballast is formed by a single integrated circuit which includes a high frequency oscillator and various control circuits. The ballast includes a compensator circuit which in response to the detection of a lower (dim) input voltage runs the oscillator at higher frequency. The higher frequency compensates for a reduced filament heating voltage and causes the lamps to remain lit at a lower intensity. While the inverter frequency changes to keep the lamps from extinguishing, dimming is primarily the result of the reduced input voltage to the inverter, and consequently is another form of voltage dimming. A disadvantage of this circuit is that the high frequency drive to the lamps is modulated on the 60 Hz AC input waveform. The ballast will have a high crest factor and power factor.
U.S. Pat. No. 4,449,897 (Sairanen) discloses yet another two wire dimming ballast. Sairanen's ballast has an EMI filter and discloses that at low conduction angles of the phase control input there is a danger that the EMI filter can cause misfiring of the triac in the phase angle controller when the EMI filter is not properly loaded. There is also the problem of an over voltage on the input buffer capacitor which feeds the inverter. To overcome this problem, Sairanen includes a switch which switches out the buffer capacitor when the lamp current is at low levels. As with the previous two-wire ballasts, the lamps are dimmed as a result of the reduced average input voltage caused by the triac dimmer.
U.S. Pat. No. 5,101,142 (Chatfield) discloses an electronic ballast for a compact fluorescent lamp with a high frequency inverter and a pre-conditioner circuit that provides active power factor correction. In the absence of phase angle dimming, the pre-conditioner also maintains a relatively constant DC bus voltage to the inverter. Because of the active power factor correction, the ballast will have a much better power factor in the absence of phase angle dimming than in the previously mentioned ballasts which had no or only passive power factor correction. The ballast is also dimmable via an external triac dimmer. During phase angle dimming a switch is opened to inactivate an error correction loop in the pre-conditioner, which error correction loop otherwise serves to maintain the DC bus voltage relatively constant. Inactivating the error correction loop permits the DC bus voltage fed to the inverter to be reduced in response to a reduced conduction angle of the mains voltage by the triac dimmer. A dimming interface circuit also receives the reduced bus voltage and provides a dim signal to a control input of a control circuit which controls inverter switching. In response to a lower DC bus voltage, the interface circuit provides a lower dim voltage to the control input, which lowers the duty cycle of the inverter, consequently lowering the net power input to the lamp.
A disadvantage of the Chatfield ballast is that the dimming signal is dependent on the voltage output by the active power factor correction circuit. Additionally, while the external dimming function is not clearly described in the '142 patent, it is believed that the disclosed arrangement has an effective dimming range limited to conduction angles less than 90 degrees, because as the conduction angle is increased above 90 degrees, the peak voltage seen by the preconditioner stays the same. Thus, the output voltage of the preconditioner remains the same above 90 degrees and consequently the derived dimming signal would remain the same. The effective dimming range would also be limited at low conduction angles by reason of the bus voltage dropping to low levels. Since the compliance voltage (i.e. the voltage needed to keep the lamp lit) increases with increased dimming, the lowering of the bus voltage at low conduction angles will be insufficient for the inverter and output circuit to maintain the compliance voltage. Thus, at low conduction angles the lamps will tend to extinguish.
Additionally, power factor correction circuits typically have a relatively slow reaction time to line variation. A reaction time of 100 ms is exemplary of the time necessary for the output voltage of the typical preconditioner to stabilize after a change in the mains input voltage. By contrast, commercially available phase angle dimmers can be changed between their high and low settings within about 30 ms. Accordingly, the dimming signal provided at the control input of the inverter driver is noticeably delayed by the preconditioner.
Accordingly, it is the object of the invention to provide a dimming ballast for a gas discharge lamp which overcomes the above mentioned disadvantages of the prior art.